Improve client subscription implementation

[mregen]

Proposed changes

• shutdown the worker tasks with a ManualResetEvent instead of timer null check
• Breaking change: Decorate the PublishStateChanged event handler with state information:
• • Stopped/Recovered if publish times out/restarts
• Breaking change: PublishStateChange fired for every received publish response, which doesn`t really indicate a state change.
  -- new behavior: fires only if a Keep alive or republish is triggered.

Types of changes

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• Bugfix (non-breaking change which fixes an issue)
• Enhancement (non-breaking change which adds functionality)
• Test enhancement (non-breaking change to increase test coverage)
• Breaking change (fix or feature that would cause existing functionality to not work as expected, requires version increase of Nuget packages)
• Documentation Update (if none of the other choices apply)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• I have read the CONTRIBUTING doc.
• I have signed the CLA.
• I ran tests locally with my changes, all passed.
• I fixed all failing tests in the CI pipelines.
• I fixed all introduced issues with CodeQL and LGTM.
• I have added tests that prove my fix is effective or that my feature works and increased code coverage.
• I have added necessary documentation (if appropriate).
• Any dependent changes have been merged and published in downstream modules.

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...

[marcschier]

Reconsider LastOrDefault as Linq is not very efficient, not sure how often this is called.

[codecov]

Codecov Report

Merging #2245 (957d7b2) into master (81340c3) will increase coverage by 0.08%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master    #2245      +/-   ##
==========================================
+ Coverage   58.90%   58.99%   +0.08%     
==========================================
  Files         328      328              
  Lines       62567    62577      +10     
==========================================
+ Hits        36854    36916      +62     
+ Misses      25713    25661      -52     

Files Changed	Coverage Δ
Libraries/Opc.Ua.Client/Subscription.cs	75.72% <83.33%> (+0.85%)	⬆️

... and 17 files with indirect coverage changes